I sold my 300D which I had used for a year, and bought the 20D. The image quality is basically the same, the extra 2 MP don't really make much practical difference for the kind of pictures that I take at this point. But it's nice to have the higher resolution for the the future.
The images I take with the camera are not really distinguishable from the results I got with the 300D. I generally use the Parameter 1 setting for more saturated/sharpened JPEGs, and occaisionally RAW mode for special shots.
The feel of the 20D is obviously more solid and hefty than the 300D. It is a little better balanced feel with a large lens like like 28-135.
For me, the instant power-up was the best thing about this camera. I just leave it powered on with 1 min shutoff time now, and I can pick it up and shoot right away without that agonizing delay on the DRebel.
I haven't done any tests, but I swear that at a given ISO this camera has slower shutter speeds for my lenses than the DRebel did . I shoot indoors a lot, and I used to get away with the 50mm/1.8 with no flash in cases when I don't seem to be able to any more. I am probably just imaging things here, but that's how it seems.